Apparatus for cleaning a charcoal canister in an evaporative emission system of an internal combustion engine is disclosed in EPO 585,527 B1. This apparatus consists of a line connecting the charcoal canister to the air intake manifold of the internal combustion engine downstream of a choke device. Thereby, ambient air can be suctioned through the canister to wash the filter therein of trapped fuel vapor and convey the same to the fuel mixture in the intake manifold. Due to fluctuating suction pressures caused by varying engine load, an irregular cleaning is obtained, which is compensated by a time controlled electromagnetic valve. A considerable cost is necessary for technical equipment and control means to operate this apparatus.
In several states in the U.S.A., an automatic monitoring of all exhaust-relevant components of a motor vehicle engine has been required (OBDII). All functions and components of engine control must be regularly monitored for their action and for their effectiveness. In addition to inspecting the ignition system for misfiring and the catalytic converter, it is also necessary to test for leak-tightness of the fuel tank system i.e. the fuel tank and the hoses of the evaporative emission system. This requirement is intended to prevent leakage of fuel vapor, for example, when the vehicle is stopped in the sun and the engine is shut off, due to poor sealing in the hose connections or due to holes in the hoses. Environmental requirements in California make it necessary for the entire fuel tank system to be tested for leak-tightness during every vehicle trip. A 1 mm hole is defined as the identification threshold.
DE 4,328,090 C2 discloses apparatus to apply suction pressure to the tank by means of manifold vacuum via a tank bleeder valve and to measure any increase in suction pressure with a pressure sensor in the tank. Difficulties in this method, arise firstly from different fuel levels in the tank which must be taken into consideration in the evaluation algorithm, and secondly, because leak-tightness testing with the suction in the intake manifold has the additional disadvantage that in order to evacuate the tank, fuel vapors must be drawn into the intake manifold via the tank bleeder valve. During travel, the fuel in the tank heats up due to injection valve operation and tends to build up fuel vapors and increased pressure. Therefore, leak-tightness testing should occur when starting a cold engine, in order to avoid measurements with distorted values due to the heated fuel. The utilization of this process is thus not suitable for testing in the case of a hot engine.
A proposal is known from PCT-WO96/14505, in which a diaphragm pump, which is driven by suction in the intake manifold, pressurizes the tank system, whose pressure drop is measured after turning off the diaphragm pump to determine lack of leak-tightness. This publication, however, also discloses aspirating the cleaning air into the suction region of the air intake system, which has the disadvantage mentioned above of irregular feed and high costs for technical equipment and control means.